Magnetic wedge

ABSTRACT

A magnetic wedge is made by preparing a composition of a thermosetting resin system and magnetizable particles, placing the composition in a mold, vibrating the mold at least until air bubbles are no longer evolved from the composition, curing the composition, and fabricating the lower portion of the cured composition into the shape desired for the wedge. The vibration removes air and compacts the particles which results in a wedge having overall superior physical and magnetic properties.

This is a division of application Ser. No. 219,663, filed Jan. 21, 1972,now U.S. Pat. No. 3,976,902.

BACKGROUND OF THE INVENTION

In the manufacture of motors and generators, coils of wire are placed inslots in laminated iron cores. These coils are held in place by wedgeswhich are inserted over the coils in the slots. Although nonmetallicfiber reinforced wedges have been used, magnetic wedges can increase theefficiency of the motor or generator by reducing core loss, temperature,frame size, and number of slots, and increasing the starting torque.(See U.S. Pat. Nos. 1,605,112 and 1,231,588). Magnetic wedges have beenmade from powdered iron and epoxy resins, (see U.S. Pat. No. 3,447,009)but until now it has been difficult to achieve both good physicalproperties and good magnetic properties in these wedges.

Good physical properties include high flexural strength since the wedgemust withstand considerable stress when it is rammed into the slot andwhen the motor or generator is running. Also, the deflection temperatureshould be high since considerable heat is generated. Good magneticproperties include high permeability and low magnetic retention. Thecore loss, which results from eddy currents and other causes should alsobe low.

SUMMARY OF THE INVENTION

I have found that magnetic wedges having superior physical and magneticproperties can be prepared by vibrating a mold containing a compositionof a thermosetting resin system and magnetizable particles. After airbubbles are no longer evolved the composition is cured and the lowerportion is shaped into the wedge. The wedges of this invention have ahigher flexural strength and a lower weight loss after 100 hours at 180°C. than do prior art wedges. Also, they have higher magneticpermeabilities, low resistance, low magnetic retention, and result inlow core loss.

THE DRAWING

The drawing is an isometric view of a section of a core of a motor orgenerator.

A laminated iron core 1 is provided with slots 2 which have grooves 3.Coils of wire 4 wrapped in insulation 5 are placed in slots 2 and heldthere by magnetic wedges 6 which are contoured to fit into grooves 3.

THE COMPOSITION

The wedges are made from a composition of a thermosetting resin systemand a filler of magnetizable particles. If the proportion of the resinsystem is too high, a great deal of the cured resin will have to bemachined away in order to make the wedge which is an expensive operationand wastes resin unnecessarily. Therefore, the composition is preferablyat least about 75% filler (all percentages herein are by weight unlessotherwise indicated) and less than about 25% resin system. On the otherhand, if the proportion of filler is too high the composition will notflow and fill the mold, and the particles will not settle easily whenthe composition is vibrated. Therefore, the composition is preferably atat least about 15% resin system and less than about 85% filler.

The magnetizable particles are preferably iron as iron is inexpensiveand has a low magnetic retention, although nickel, cobalt, and variousmagnetic alloys could also be used. The particles should not be too fineor they will not settle in a reasonable amount of time, and shouldtherefore preferably be greater than about 0.001 mm. If the particlesare too large the interstices become large and good compaction isimpossible. Therefore, it is preferable that the particles be less thanabout 0.1 mm. Also, a mixture of sizes within the indicated range ispreferred as greater compaction can be achieved with a distribution ofparticle size rather than with particles of all the same size.

Preferably the particles are elongated (e.g. very thin wires orwhiskers) so that they can be aligned lengthwise with the length of thewedge for maximum mechanical strength. The elongated particles shouldhave a length-to-diameter ratio of at least about 50 and preferably atleast about 250 for the best flexural modulus. Typically, the elongatedparticles are about 0.0003 inches in diameter and about 0.075 incheslong.

The thermosetting resin system comprises a thermosetting resin and suchhardeners, accelerators, etc. as are necessary to cure the resin. Theviscosity of the resin system should preferably be less than about 300cps. measured at 25° C. if no solvent is present. If a solvent ispresent the viscosity of the remaining components of the resin systemmay exceed 300 cps but the resin system (including the solvent) shouldstill preferably have a viscosity less than about 300 cps. Solvents arepreferably not used as they are difficult to remove during curing. Thethermosetting resin when cured, should have a deflection temperature ofat least 150° C. to withstand the high temperatures encountered inmotors and generators. (See ASTM D648). The cured resin should also havea flexural strength of at least 10,000 psi at 25° C. so that the wedgecan withstand the stresses of asembly and use.

Polyester resins and epoxy resins are among the suitable resins. Epoxyresins are preferred because they cure slowly enough to permitadditional vibration after a partial cure and before the final cure.

The composition also preferably includes about 0.5 to about 1.5% of aflexibilizer if the resin is brittle as most epoxy resins are. Aflexibilizer reduces the brittleness and becomes part of the curedresin. The preferred flexibilizer is polypropylene glycol as it workswell and is non-toxic, but other flexibilizers, which do not appreciablylower the heat distortion temperature could also be used.

THE METHOD

The composition is prepared, preferably by mixing under a vacuum toexclude air, and is placed in a mold connected to a vibrator. The moldis in the approximate shape of one or more wedges and may be horizontalor vertical. Vertical molds offer the advantage that, after curing, itis easier to remove the resin at the top of the mold which does notcontain much filler. However, air bubbles must travel farther through avertical mold and the filler density may be greater at the bottom of themold than at the top. For these reasons, a horizontal mold is preferred.

The mold is vibrated, preferably at a frequency of about 2 to about 60cycles/sec and an amplitude of about 3 to about 6 mils, until airbubbles are no longer evolved, usually about 15 minutes. The vibrationmay be in any direction, including a circular vibration. If theparticles are elongated the vibration is preferably done in a magneticfield, with the poles of the field at the ends of the mold. The magneticfield aligns the particles in the direction of the length of the wedgefor maximum physical strength. A uniform field of at least about 10gauss should be used and preferably of at least about 100 gauss forfaster alignment of the particles.

The mold is then placed in an oven and cured at the curing temperatureof the particular resin used. If an epoxy resin is used, it ispreferably to heat composition at about 80° C. to about 100° C. forabout 15 minutes in order to lower its viscosity, and then vibrate themold an additional 5 minutes to remove any bubbles that may have beenevolved during the initial heating period, before finishing the cure atabout 100° C. to about 120° C. for about 45 minutes. Preferably, thecomposition is then cured at about 120° C. to about 200° C. for about 4hours, and cooled slowly in an oven to prevent warping. Alternatively,the mold could be vibrated during the cure.

The resin on the upper portion, usually not greater than the upper twofifths, which contains little filler, is removed, for example bygrinding. The piece is cut to the approximate size of a wedge and ismachined to the exact size. Typical wedges are about 5 to 6 inches long,about 3/4 to 1 inch wide, and about 1/8 to 1/4 inch thick. About 10 to14 wedges would typically be used to fill a slot and a motor orgenerator may have about 48 slots. The wedges contain about 65 to 70% byvolume filler and if the filler is iron, they contain about 80 to 90%(by weight) iron, and preferably about 83 to 90% iron. The wedges may beused in motors and generators, but offer the greatest advantages whenused in A.C. motors.

EXAMPLE 1

The following composition was prepared, mixing under a vacuum.

    ______________________________________                                        % by weight                                                                   ______________________________________                                        8.9      alicyclic diepoxy carboxylate,                                                viscosity = 275 cps at 25° C, of the                                    ##STR1##                                                                     sold by Ciba -- Geigy CO under the trade-                                     mark "CY179"                                                         9.1      Hexahydrophthalic anhydride                                          0.9      Polypropylene glycol (mole. wt. = 425)                               1.1      Organometallic accelerator sold by Ciba --                                    Geigy Co. under the trademark "C.Y.065"                              80.0     Iron particles sold by Hoeganaes Corp.                                        under the trademark "MH 300"                                                  Sieve analysis: 4% + 230, 10% - 230 to                                        + 325, 86% - 325.                                                    ______________________________________                                    

About 10,242 gms of the composition was placed in a horizontal moldhaving dimensions 25 inches long, 25 inches wide, and 0.25 inches deep.The mold was placed on a jogger vibrator and vibrated at an amplitude of5 mils and a frequency of 60 cycles/sec for 15 minutes. It was thenplaced in an oven at 120° C. for 15 minutes which gave the composition atemperature of about 100° C. The composition was vibrated an additional5 minutes, and returned to the oven for 45 more minutes. The compositionwas cured at 150° C. for 4 hours and slowly cooled. The top of themolded piece was ground off until the piece was 0.16 inches deep. Thewedge was compared to two other commerical magnetic wedges of the samedimensions. The European wedge was an iron powder-resin material and theU.S. wedge was a randomly-dispersed glass-fiber reinforced material witha metallic filler. The following table gives the results.

    ______________________________________                                                             Weight                                                           Flexural     Loss      Re-                                            Type of Strength     After 100 sistance                                                                             Specific                                Wedge   25° C, ASTM.sub.(psi)                                                               Hours (%) (φ/cm)                                                                           Gravity                                 ______________________________________                                        This                                                                          Example 11,300       0.10      1305   3.9                                     European                                                                              6,350        0.14      --     4.0                                     U.S.    5,250        0.40      200    3.6                                     ______________________________________                                    

The above table shows that, compared to the other two commercial wedges,the wedges of this invention have a far greater flexural strength, andless weight loss after 100 hours at 180° C., which is a severe test ofdurability. The higher resistance also indicates less losses from eddycurrents.

The wedge of this example and three commercial wedges were tested on a150 amp, 2080 volt, 1185 rpm, 600hp AC motor. The following table givesthe results:

    __________________________________________________________________________    Type   No   Friction                                                                             No  Core         Reduction            Reduction            of     Load and Wind-                                                                            Load                                                                              Loss Magnetizing                                                                           in Core                                                                             Locked                                                                             Locked                                                                             Starting                                                                           in Start-            Wedge  (KW) age (KW)                                                                             I.sup.2 R                                                                         (KW) Amps    Loss  kva  kw   Torque                                                                             ing                  __________________________________________________________________________                                                             Torque               This   13.0  6.0   0.5 6.5  40.2    -46.5%                                                                              66.0 628 70.0  -5%                  Example                                                                       European                                                                             12.4  6.15  0.48                                                                              5.77 41.0    -48%  65.7 657 69.5  -6%                  U.S.   14.6  6.0   0.5 8.10 44.0    -27%  66.8 736 79.0  +7%                  Non-   16.64 4.9   0.64                                                                              11.1 47.2     0    67.4 757 75.7  0                    Magnetic                                                                      __________________________________________________________________________     The above table shows that the wedge of this example compared favorably       with commercial wedges in magnetic and electrical properties.            

The wedge was placed in the center of an air core transformer and theresulting increase in flux linkages was measured. The test showed thatat an applied force at 1000 ampere turns per inch about 26.25 kilolinesof flux per square inch were generated. This figure was higher than thatobtained for any other commercial magnetic wedge or for any otherexperimental wedge tested. It indicates very high permeability andefficiency.

EXAMPLE 2

Example 1 is repeated except that the iron particles are elongated andhave a length to diameter ratio of not less than 50. The results aresimilar except that a higher flexural strength is obtained.

I claim:
 1. A magnetic wedge comprising a cured thermosetting resinsystem containing about 65 to about 75% by volume elongated magnetizableparticles orientated in the direction of said wedge and having a lengthto diameter ratio of at least about
 250. 2. A magnetic wedge accordingto claim 1 wherein said particles are iron.
 3. A magnetic wedgeaccording to claim 2 wherein said particles are about 80 to about 90% byweight of said wedge.
 4. A magnetic wedge according to claim 12 whereinsaid thermosetting resin system comprises a resin selected from thegroup consisting of epoxy resins and polyester resins.
 5. A magneticwedge according to claim 1 wherein said thermosetting resin systemcomprises an epoxy resin.
 6. A magnetic wedge according to claim 1wherein said resin system includes about 0.5 to about 1.5% of aflexibilizer.
 7. A magnetic wedge according to claim 6 wherein saidflexibilizer is polypropylene glycol.
 8. A magnetic wedge according toclaim 1 wherein said particles are iron.
 9. A magnetic wedge accordingto claim 1 wherein said elongated particles are about 0.0003 inches indiameter and about 0.075 inches long.